Why union types are needed in Typescript - See It in Action

Why union types are needed

📖 Scenario: Imagine you are building a simple app that accepts user input. Sometimes the input is a number, and sometimes it is a word. You want to write code that can handle both cases safely.

🎯 Goal: You will create a variable that can hold either a number or a string using union types. Then you will write code to check the type and handle each case properly.

📋 What You'll Learn

Create a variable called userInput that can hold a number or a string

Create a variable called userInput and assign a number to it

Write an if statement that checks if userInput is a string

Print different messages depending on whether userInput is a string or a number

💡 Why This Matters

🌍 Real World

Many apps accept user input that can be different types, like numbers or text. Union types help handle these safely.

💼 Career

Understanding union types is important for writing flexible and error-free TypeScript code in real projects.

1

Create a union type variable

Create a variable called userInput that can hold either a number or a string. Assign the number 42 to it.

Typescript

// Your code here

Hint

Use the | symbol to combine types in TypeScript.

2

Assign a new value to the union type variable

Assign the string "hello" to the variable userInput.

Typescript

let userInput: number | string = 42;
// Your code here

Hint

You can assign either a number or a string to userInput.

3

Check the type of the union variable

Write an if statement that checks if userInput is a string using typeof userInput === "string".

Typescript

let userInput: number | string = 42;
userInput = "hello";

// Your code here

Hint

Use typeof to check the type inside the if statement.

4

Print the result based on the type

Run the code and observe the output. It should print User input is a string: hello.

Typescript

let userInput: number | string = 42;
userInput = "hello";

if (typeof userInput === "string") {
    console.log("User input is a string: " + userInput);
} else {
    console.log("User input is a number: " + userInput);
}

// Your code here

Hint

Check the console output to see the message.

Practice

1. Why do we need union types in TypeScript?
let value: string | number;

easy

A. To make variables immutable

B. To allow a variable to hold more than one type of value

C. To force variables to be only one type

D. To automatically convert types at runtime

Solution

1. Step 1: Understand union type declaration

   The syntax string | number means the variable can hold either a string or a number.

2. Step 2: Identify the purpose of union types

   This flexibility allows one variable to store multiple types safely without errors.

3. Final Answer:

   To allow a variable to hold more than one type of value -> Option B

4. Quick Check:

   Union types = multiple allowed types [OK]

Hint: Union types let variables hold multiple types safely [OK]

Common Mistakes:

• Thinking union types make variables immutable
• Believing union types force a single type only
• Confusing union types with automatic type conversion

2. Which of the following is the correct syntax to declare a variable that can be a string or number in TypeScript?

easy

A. let data: string & number;

B. let data: string or number;

C. let data: string | number;

D. let data: string, number;

Solution

1. Step 1: Recall union type syntax

   Union types use the pipe symbol | to separate types.

2. Step 2: Compare options

   let data: string | number; uses | correctly, while others use invalid syntax.

3. Final Answer:

   let data: string | number; -> Option C

4. Quick Check:

   Union syntax uses | symbol [OK]

Hint: Use | to separate types in union declarations [OK]

Common Mistakes:

• Using & instead of | for union types
• Separating types with commas
• Writing 'or' instead of |

3. What will be the output of this TypeScript code?

function format(value: string | number) {
  if (typeof value === 'string') {
    return value.toUpperCase();
  } else {
    return value.toFixed(2);
  }
}

console.log(format('hello'));
console.log(format(3.14159));

medium

A. "HELLO" and 3

B. "hello" and "3.14159"

C. Error because value can be string or number

D. "HELLO" and "3.14"

Solution

1. Step 1: Analyze function behavior for string input

   If input is a string, toUpperCase() converts it to uppercase, so 'hello' becomes 'HELLO'.

2. Step 2: Analyze function behavior for number input

   If input is a number, toFixed(2) formats it to 2 decimal places, so 3.14159 becomes '3.14'.

3. Final Answer:

   "HELLO" and "3.14" -> Option D

4. Quick Check:

   String upper + number fixed decimals = output [OK]

Hint: Check type with typeof before calling type-specific methods [OK]

Common Mistakes:

• Assuming no type check needed and causing errors
• Expecting original number without formatting
• Confusing toFixed output as number instead of string

4. Identify the error in this TypeScript code using union types:

function printLength(value: string | number) {
  console.log(value.length);
}
printLength('test');
printLength(123);

medium

A. Error because number type has no 'length' property

B. Error because string type has no 'length' property

C. No error, code runs fine

D. Error because union types cannot be used in functions

Solution

1. Step 1: Understand property access on union types

   Accessing length on value is unsafe because numbers don't have a length property.

2. Step 2: Identify the error cause

   TypeScript will raise an error to prevent runtime failure when value is a number.

3. Final Answer:

   Error because number type has no 'length' property -> Option A

4. Quick Check:

   Accessing length on number causes error [OK]

Hint: Check type before accessing properties on union types [OK]

Common Mistakes:

• Ignoring type checks and expecting no error
• Thinking string has no length property
• Believing union types disallow functions

5. You want a function that accepts either a string or an Array<string> and returns the total number of characters. Which is the best way to use union types to achieve this?

hard

A. function countChars(input: string | string[]) {
if (typeof input === 'string') {
return input.length;
} else {
return input.join('').length;
}
}

B. function countChars(input: string & string[]) {
return input.length;
}

C. function countChars(input: string | string[]) {
return input.length;
}

D. function countChars(input: any) {
return input.length;
}

Solution

1. Step 1: Understand input types and their length

   A string has a length property; an array of strings also has length, but we want total characters, so we join the array first.

2. Step 2: Check union type handling

   function countChars(input: string | string[]) { if (typeof input === 'string') { return input.length; } else { return input.join('').length; } } uses a type check to handle string and array separately, joining array elements to count total characters correctly.

3. Final Answer:

   function countChars(input: string | string[]) { if (typeof input === 'string') { return input.length; } else { return input.join('').length; } } -> Option A

4. Quick Check:

   Use type check and join array for total chars [OK]

Hint: Use typeof and array methods to handle union types properly [OK]

Common Mistakes:

• Using intersection (&) instead of union (|)
• Not checking type before accessing properties
• Using any type losing type safety